The present invention relates to mitigation of false co-channel uplink reception (also known as xe2x80x9cshow-thruxe2x80x9d) in a satellite communication system. In particular, the present invention relates to staggering data transmissions in uplink beams to allow a receiving satellite to reliably discard show-thru signals.
A multi-beam satellite system may transmit and receive numerous beams distinguished by the frequency and polarization of the signals present in the beam. The same frequency, same polarization beams (i.e., the same xe2x80x9ccolorxe2x80x9d beams) may be reused in an antenna pattern to provide communications services across a coverage area often hundreds of miles in diameter.
Beam reuse, however, renders the same color beams susceptible to co-channel interference such that a portion of the signal from a beam B is observed in a beam A. Such co-channel interference (CCI) arises primarily because of the practical limitations in implementing an antenna system which, ideally, would provide perfect rejection of same-color signals from other than the desired beam. In practice, however, the coverage provided for beam A inevitably provides some response to signals originating in beam B due to real world limitations in the physical realization of the antenna system. Furthermore, in typical frequency reuse systems, 12, 16, or more beams of the same color may be reused over a coverage area, correspondingly increasing the potential for CCI in any one of the beams.
Processing satellite systems may further employ TDMA (time division multiple access) techniques that permit several user earth terminals to time share a frequency channel by sending short traffic bursts in assigned time slots. These bursts typically exploit powerful error correcting codes to ensure the integrity of the traffic being carried. However, when the time slots in two same-color beams, A and B, are time aligned, a phenomenon known as show-thru may occur.
Specifically, show-thru may occur in beam A when no burst is present in a time slot in beam A but a burst is present in the matching channel (frequency) and time slot in beam B. As a result of CCI, the burst from beam B couples (albeit in attenuated form) into the receiving electronics for beam A. Although the signal to noise ratio of this inadvertently coupled signal may be quite low, the error correcting code applied to the burst in beam B improves the likelihood that the burst from B will be regarded as valid by the processor for beam A.
In systems intended for use with the ATM (Asynchronous Transfer Mode) protocol, the inadvertent presence of apparently valid, but actually misinserted cells (due to show-thru) is deleterious since the cell misinsertion rate (CMR) must be kept very low to minimize the potential for confusion at higher layers of the ATM communications protocol. Furthermore, show-thru presents, in general, a security threat to the information in the uplink burst for beam B. In other words, neighboring quiescent channels may actually decode (and ultimately send to unintended recipients) an uplink burst intended for a completely different receiver. Show-thru thus undesirably and unnecessarily utilizes satellite resources, permits unauthorized usage of uplink beams, and compromises the security in the transmitted data.
Therefore, a need is present in the industry for an improved satellite communications system which overcomes the disadvantages discussed above and previously experienced.
It is an object of the present invention to greatly reduce the likelihood of successfully decoding show-thru signals.
Another object of the present invention is to provide a simple and effective method of reliably discarding show-thru signals at a receiver.
It is another object of the present invention to greatly reduce the likelihood of successfully decoding show-thru signals without adding overhead to uplink bursts.
Yet another object of the present invention is to use the cyclic nature of error correcting codes in a staggered transmission system to reliably discard show-thru signals at a receiver.
One or more of the foregoing objects is met in whole or in part by a method and apparatus for preventing show-thru among uplink beams transmitted to a satellite. The method includes the steps of selecting an uplink A stagger value, selecting an uplink B stagger value, coding uplink A data to generate coded uplink A data, and coding uplink B data to generate coded uplink B data. Subsequently, the method transmits the coded uplink A data, staggered by the uplink A stagger value, in the uplink A and further transmits the coded uplink B data, staggered by the uplink B stagger value, in the uplink B.
Additionally, a predetermined set of staggering values for each same color uplink beam in a predetermined reuse plan may be generated. The staggering values may then be distributed among the user earth terminals generating the uplinks in the reuse plan. During coding, the method may use a full length error correcting code capable of correcting t errors. In such a case, the method generally selects an uplink B stagger value differing from the uplink A stagger value by more than t codeword symbols. For example, the uplink A stagger value may be zero and the uplink B stagger value may be t+1.
Alternatively, the method, may use a shortened error correcting code capable of correcting t errors. In such a case, the method generally selects an uplink B stagger value differing from the uplink A stagger value by at least one codeword symbol. However, a difference in staggering value of more than t codeword symbols is also appropriate. The method is not limited to any particular number of uplink beams and thus, for example, the method may also select an uplink C stagger value differing from the uplink A stagger value and differing from the uplink B stagger value, code the uplink C data, and transmit the uplink C data staggered by the uplink C stagger value, in a separate uplink C.